1. Field of the Invention
The invention relates to a radiation-reflective apparatus for optically scanning an information plane, which apparatus comprises a diode laser supplying a scanning beam, an objective system for focussing the scanning beam to form a scanning spot on the information plane, a composite diffraction grating which is arranged between the diode laser and the objective system, which grating comprises two sub-gratings for deflecting a radiation beam reflected by the information plane to a radiation-sensitive detection system comprising two detector pairs, and for splitting said radiation beam into two sub-beams each co-operating with a separate detector pair.
2. Description of the Related Art
An apparatus of this type which is in principle suitable for reading a prerecorded record carrier and for optically recording on such a record carrier is known from U.S. Pat. No. 4,665,310 (PHN 11.531). In this apparatus the composite diffraction grating performs two functions for which otherwise two separate elements are required. Firstly, the grating ensures that the radiation which has been reflected by the information plane and which traverses the objective system is deflected from the path of the radiation emitted by the diode laser so that a detection system can be arranged in the path of the reflected radiation. Secondly, the grating splits the reflected beam into two sub-beams required for generating a focussing error signal, i.e. a signal containing information about the magnitude and the direction of a deviation between the focal plane of the objective system and the information plane. Each of the sub-beams is associated with a separate detector pair, the signal representing the difference between the output signals of the detectors of the same pair being a measure of the focussing of the canning beam on the information plane.
In the said record carrier the information is arranged in accordance with information tracks. If the bounding line between the two sub-gratings extends parallel to the track direction, it is possible, by determining the sum of the output signals of each detector pair and sub-tracting these sum signals from each other, to form a signal containing information about the magnitude and the direction of a deviation between the centre of the scanning spot and the central axis of the information track to be scanned.
In order to obtain the desired beam splitting the diffraction grating in the known apparatus comprises two sub-gratings having the same grating period, whilst the grating strips of the first sub-grating extend at a first angle and the grating strips of the second sub-grating extend at a second angle, which is equal but opposite to the first angle, to the bounding line of between the two sub-gratings. Since a diffraction grating deflects an incident beam in a plane transversely to the direction of the grating lines, the part of the beam which is incident on one of the sub-gratings will be given a different direction than the part of the beam which is incident on the second sub-grating.
There is an increasing need to reduce the dimensions of optical scanning apparatus for optical record carriers such as the known "CD players" so that these apparatus can be, for example, built in more easily. A reduction of the optical path length between the diode laser and the record carrier is of special importance. This length can be reduced if the distance between the diode laser and the diffraction grating can be reduced. When reducing this distance, the distance between the diffraction grating and the imaging lens system can also be reduced, whilst complying with the requirement that the first and higher order parts of the diffracted laser beam fall outside the pupil of this lens system. When assembling the apparatus it must be possible to accurately adjust the distance, measured in a direction parallel to the optical axis of the apparatus, between the diode laser and the detectors because otherwise an offset in the focussing error signal is produced so that the scanning beam is no longer focussed on the information plane.
In aiming at less expensive, lighter and smaller scanning apparatus it is convenient to make use of an element supplied by, for example, a semiconductor component manufacturer, which element accommodates a diode laser and detectors in the form of photodiodes, which diode laser and photodiodes are fixed with respect to each other. Due to manufacturing tolerances allowance must be made for the fact that the said distance deviates from the desired distance resulting in the offset in the focussing error signal. The influence of the said deviation will be greater as the distance between the diffraction grating and the diode laser is smaller.
The present invention provides the possibility of compensating for the focus offset in the scanning apparatus introduced as a result of an incorrect distance between the diode laser and the photodiodes in the direction of the optical axis.
As set forth in U.S. Pat. No. 4,665,310 the grating design described in this patent is based on a previously proposed composite diffraction grating. This grating comprises two sub-gratings in which the grating strips of the one sub-grating have the same direction as those of the other sub-grating, but in which the grating periods of the two sub-gratings are different. In an apparatus comprising such a composite grating the two sub-beams derived from the reflected beam are focussed in radiation spots located on a curve in a plane perpendicular to the plane of the detectors. Consequently, if the detector pairs are located in one plane, the two radiation spots cannot be focussed equally sharply with respect to their associated detector pair. An offset in the focussing error signal is therefore even sooner produced in this device than in a device with a composite diffraction grating in which the grating strips of the one sub-grating extend at an angle to those of the other sub-grating.
Consequently, and also for reasons to be stated hereinafter, the present invention is eminently suitable for use in an apparatus comprising a diffraction grating which consists of two sub-gratings whose grating lines are parallel.